The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased “octane,” or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds. Such key options as increased reforming severity and higher FCC gasoline octane result in a higher aromatics content of the gasoline pool at the expense of low-octane heavy paraffins.
Refiners are also faced with supplying reformulated gasoline to meet tightened automotive emission standards. Reformulated gasoline differs from the traditional product in having a lower vapor pressure, lower final boiling point, increased content of oxygenates, and lower content of olefins, benzene and aromatics. Benzene content generally is being restricted to 1% or lower, and is limited to 0.8% in U.S. reformulated gasoline. Gasoline aromatics content is likely to be lowered, particularly as distillation end points (usually characterized as the 90% distillation temperature) are lowered, since the high-boiling portion of the gasoline which thereby would be eliminated usually is an aromatics concentrate. Since aromatics have been the principal source of increased gasoline octanes during the recent lead-reduction program, severe restriction of the benzene/aromatics content and high-boiling portion will present refiners with processing problems. These problems have been addressed through such technology as isomerization of light naphtha to increase its octane number, isomerization of butanes as alkylation feedstock, and generation of additional light olefins as feedstock for alkylation and production of oxygenates using FCC and dehydrogenation. This issue often has been addressed by raising the cut point between light and heavy naphtha, increasing the relative quantity of naphtha to an isomerization unit.
Additionally, instead of reforming, the isomerization of longer chain hydrocarbons such as C7 and C8 hydrocarbons into branched hydrocarbons of higher octane could be used to increase the octane number of fuels without increasing the amount of aromatics. However, many isomerization catalysts suffer significant disadvantages when applied to the longer chain hydrocarbons. A principal problem is the generation of byproducts such as cracked hydrocarbon materials. The cracking decreases the amount of long chain paraffins available for isomerization and reduces the ultimate yield.
Several catalysts for isomerization are known, and a family of tungstated zirconia catalysts have been used. For example, U.S. Pat. Nos. 5,510,309 B1, 5,780,382 B1, 5,854,170, and 6,124,232 B1 teach methods of making an acidic solid having a Group IVB (IUPAC 4) metal oxide modified with an oxyanion of a Group VIB (IUPAC 6) metal such as zirconia modified with tungstate. U.S. Pat. No. 6,184,430 B1 teaches a method of cracking a feedstock by contacting the feedstock with a metal-promoted anion modified metal oxide catalyst where the metal oxide is one or more of ZrO2, HfO2, TiO2 and SnO2, the modifier is one or more of SO4 and WO3, and the metal is one or more of Pt, Ni, Pd, Rh, Ir, Ru, Mn, and Fe.
Others have added a noble metal such as platinum to the tungstated zirconia catalysts above, see U.S. Pat. Nos. 5,719,097; 6,080,904 B1; and 6,118,036 B1. A catal having an oxide of a Group IVB (IUPAC 4) metal modified with an anion or oxyanion of a Group VIB (IUPAC 6) metal and a Group IB (IUPAC 11) metal or metal oxide is disclosed in U.S. Pat. No. 5,902,767. In U.S. Pat. Nos. 5,648,589 and 5,422,327, a catalyst having a Group VIII (IUPAC 8, 9, and 10) metal and a zirconia support impregnated with silica and tungsten oxide and a process of isomerization using the catalyst is disclosed. A process for forming a diesel fuel blending component uses an acidic solid catalyst having a Group IVB (IUPAC 4) metal oxide modified with an oxyanion of Group VIB (IUPAC 6) metal and iron or manganese in U.S. Pat. No. 5,780,703 B 1.
U.S. Pat. Nos. 5,310,868 and 5,214,017 teach catalyst compositions containing sulfated and calcined mixtures of (1) a support containing an oxide or hydroxide of IUPAC 4 (Ti, Zr, Hf), (2) an oxide or hydroxide of IUPAC 6 (Cr, Mo, W); IUPAC 7 (Mn, Tc, Re), or IUPAC 8, 9, and 10 (Group VIII) metal, (3) an oxide or hydroxide of IUPAC 11 (Cu, Ag, Au), IUPAC 12 (Zn, Cd, Hg), IUPAC 3 (Sc, Y), IUPAC 13 (B, Al, Ga, In, Tl), IUPAC 14 (Ge, Sn, Pb), IUPAC 5 (V, Nb, Ta), or IUPAC 6 (Cr, Mo, W), and (4) a metal of the lanthanide series.
U.S. Pat. No. 5,489,733 teaches a catalyst having a zirconium hydroxide support, a Group VIII metal, and a heteropolyacid selected from the group consisting of the exchanged aluminum salt of 12-tungstophosphoric acid, the exchanged salt of 12-tungstosilicic acid, and mixtures thereof. The catalyst is used for isomerization processes having a feed comprising Cn or Cn+ wherein n=4.
Applicant has developed a more effective catalyst that has proved to be surprisingly superior to those already known for the isomerization of hydrocarbons and especially C7 and C8 hydrocarbons.